Electric power steering apparatus

ABSTRACT

A controller for controlling the assist steering torque of an electric motor in an electric power steering apparatus has a relay failure detection unit for detecting ON failure which arises in a relay circuit provided between an electric motor driving circuit and a power supply; and a relay failure deal correction unit for loweringly correcting the assist steering torque, at least the maximum value of the assist steering torque, of the electric motor based on a failure detection signal from the relay failure detection unit. Thereby, manual steering by a steering wheel with a heavier steering torque than the torque at the time of a normal mode becomes possible as deal at the time that ON failure arises in a relay circuit.

FIELD OF THE INVENTION

[0001] This invention relates to an electric power steering apparatus for applying an assist steering torque to the steering system of a vehicle by an electric motor, and more particularly to an electric power steering apparatus to improve deal at the time of ON failure in a relay circuit provided between a driving circuit of the electric motor and a power supply.

BACKGROUND OF THE INVENTION

[0002] In recent years, an apparatus for reducing the steering force of a driver by applying an assist steering torque to a steering system by an electric motor when a steering wheel is operated, a so-called electric power steering apparatus gradually spreads as the steering apparatus of a vehicle.

[0003] An electric power steering apparatus, basically, comprises a steering torque sensor for detecting the steering torque of the steering system with generation of a steering wheel, a controller for outputting a control signal according to the direction and magnitude of a detection torque, and an electric motor driving circuit for driving the electric motor according to the control signal from the controller.

[0004] In the electric power steering apparatus, a relay circuit is provided between the electric motor driving circuit and a power supply. This relay circuit is circuit configured such that ON/OFF is available by the instruction of the controller and the relay circuit becomes OFF when the controller etc. fails (See Japanese Utility Model publication No.H5-42705).

[0005] However, failure in which joint melts and becomes stuck, so-called ON failure, sometimes arises in the relay circuit. In this case, the electric power steering apparatus exerts a function in a case where the electric motor driving circuit is in a normal mode. However, there is fear of causing a problem in a case where failure arises in the electric motor driving circuit in this state, a problem in which excess current is sent to the electric motor or the power supply circuit is a short circuit or the like in a state where failure arises in the electric motor driving circuit.

[0006] So, in a conventional electric power steering apparatus, occurrence of failure in the relay circuit is checked as an initial check at the time that the ignition switch turns ON and the function of the electric power steering apparatus is instantaneously stopped as the electric motor driving circuit being the OFF state.

[0007] Note that, once the function of the electric power steering apparatus is instantaneously stopped at the time of ON failure of the relay circuit, manual steering torque of the steering wheel becomes extremely heavy due to a lack of the assist steering torque such as the conventional electric power steering apparatus. In particular, great manual steering torque is required for a vehicle of a large load applied to a front shaft, and thereby manual steering by the steering wheel becomes difficult and there is fear of causing trouble for driving the vehicle.

SUMMARY OF THE INVENTION

[0008] This invention has been made to solve the above problems, and it is there fore the object of the invention to provide an electric power steering apparatus which is capable of manual steering by a steering wheel with heavier torque than the torque at the time of normal as deal at the time of ON failure in a relay circuit. Another object of the invention is to provide an electric power steering apparatus which comprises:

[0009] (1) an electric motor for applying an assist steering torque to the steering system of a vehicle;

[0010] (2) an electric motor driving circuit for driving the electric motor;

[0011] (3) a steering torque sensor for detecting the steering torque of the steering system;

[0012] (4) a controller for outputting a control signal based on a detection signal of the steering torque sensor to the electric motor driving circuit to control an assist steering torque with the electric motor; and

[0013] (5) a relay circuit, which is capable of switching ON/OFF according to instructions of the controller, which is provided between the electric motor driving circuit and a power supply, wherein the controller includes

[0014] (a) a relay failure detection unit for detecting ON failure in the relay circuit; and

[0015] (b) a relay failure deal correction unit for loweringly correcting the assist steering torque, at least the maximum value of the assist steering torque, of the electric motor based on the failure detection signal of the relay failure detection unit at the time of ON failure of the relay circuit.

[0016] In the electric power steering apparatus of the present invention, the relay circuit turns ON according to the instruction of the controller, and connects the electric motor driving circuit and the electric motor to a power supply. And the steering torque sensor detects the steering torque of the steering system with operation of the steering wheel and outputs the control signal based on this steering torque to the electric motor driving circuit to control the assist steering torque applied to the steering system by the electric motor. When the relay failure detection unit detects ON failure of the relay circuit, the relay failure deal correction unit loweringly corrects the maximum value of the assist steering torque by the electric motor based on the failure detection signal. As a result of this, although the steering torque by the steering wheel becomes heavier than the steering torque at the time of normal mode, manual steering becomes possible even for the vehicle of the large load applied to the front shaft.

[0017] The steering torque of the steering wheel becomes gradually heavier for every restart of the vehicle after ON failure of the relay circuit in a case where the relay failure deal correction unit is configured so as to gradually increase the amount of the loweringly corrected assist steering torque according to the number of accumulation input times of the ON signal of an ignition switch after ON failure arises in the relay circuit in the electric power steering apparatus of the present invention.

[0018] In the electric power steering apparatus of the present invention, the relay failure deal correction unit may be configured so as to multiply a signal value, the signal value being set with the controller, by a correction coefficient having a decimal value, which is less than one, to loweringly correct the maximum value of the assist steering torque by the electric motor based on the detection signal of the steering torque sensor. The relay failure deal correction unit may be configured so as to loweringly correct the maximum value of the assist steering torque within a limit of the maximum value of the signal value.

[0019] The above and other objects, features, and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a configuration diagram of the steering system applied for an electric power steering apparatus according to one embodiment of the present invention.

[0021]FIG. 2 is a block configuration diagram of the electric power steering apparatus.

[0022]FIG. 3 is a circuit diagram of a bridge circuit in the electric power steering apparatus.

[0023]FIG. 4 is a block configuration diagram of a relay failure deal correction unit and a relay failure detection unit in the electric power steering apparatus.

[0024]FIG. 5 is a flowchart showing data proceeding in the relay failure deal correction unit and the relay failure detection unit of the electric power steering apparatus.

[0025]FIG. 6 is a block configuration diagram of the relay failure deal correction unit and relay failure detection unit in the electric power steering apparatus according to another embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Below, one embodiment of electric power steering apparatus according to present invention will be described referring to drawings. In drawings, FIG. 1 is configuration diagram of steering system to which electric power steering apparatus is applied and FIG. 2 is block diagram of electric power steering apparatus.

[0027] In explaining electric power steering apparatus, configuration of steering system to which electric power steering apparatus is applied is described by referring to FIG. 1. Steering system is steering system of so-called rack/pinion type, and bottom end portion of steering shaft 2 integrally connected to steering wheel 1 is coupled with input shaft 5A of steering torque sensor 5 via pair of universal joints 4,4 connected mutually by coupling shaft 3. And pinion 6A of rack pinion mechanism 6 is integrated with output shaft of steering torque sensor 5.

[0028] Rack pinion mechanism 6 has rack shaft 6C in which rack gear 6B is formed, rack gear 6B being engaged with pinion 6A, and knuckle arms (not shown) set to right and left front wheels 7,7 of vehicle are respectively coupled with both end portions of rack shaft 6C via tie rods 8,8. And, ball screw portion 9A of ball screw mechanism 9 is formed in rack shaft 6C. Ball nut 9B engaged with ball screw portion 9A is fixed to rotor 10A of electric motor 10 and electric motor 10 covers circumference of rack shaft 6C which penetrates electric motor 10.

[0029] Electric power steering apparatus comprises vehicle speed sensor 11 for detecting number of speeds of unshown transmission output shaft, controller 12 for inputting at least detection signals of vehicle speed sensor 11 and steering torque sensor 5, electric motor driving circuit 13 for inputting control signal from controller 12, current sensor 14 for detecting current supplied from electric motor driving circuit 13 to electric motor 10 to output detected signal to controller 12, and ignition switch 15 for outputting ignition signal IG according to ON/OFF operation of ignition key.

[0030] First, each sensor is described. Steering torque sensor 5 detects direction and magnitude of steering torque according to screwed angle between input shaft 5A and pinion 6A, which is output shaft and outputs steering torque signal TS, which is detection signal to controller 12. And speed sensor 11 outputs vehicle speed signal VP according to number of speeds of transmission output shaft (not shown) to controller 12 as digital signal. Further, current sensor 14 has resistance connected in series with electric motor 10 comprising direct current servomotor or hall element and outputs motor current signal IM according to direction and magnitude of current sent to electric motor 10. And ignition switch 15 is output ignition with ON/OFF signal according to ON/OFF operation of ignition key.

[0031] Next, controller 12 and electric motor driving circuit 13 are described. Controller 12 has input/output interface I/O among steering torque sensor 5, vehicle speed sensor 11, current sensor 14, and ignition switch 15, A/D converter for converting analog signal input from these sensors into digital signal, ROM (Read Only Memory) for storing various kinds of data or programs, RAM (Random Access Memory) for temporary storage of various kinds of data etc., and CPU (Central Processing Unit) for performing various kinds of calculation procedures etc. as hardware.

[0032] Controller 12 includes target current value set unit 12A, relay failure deal correction unit 12B, deviation calculation unit 12C, PID (Proportional Integral Differential) control unit 12D, PWM (Pulse Width Modulation) signal generation unit 12E, and relay failure detection unit 12F. Electric motor driving circuit 13 includes gate driving circuit 13A and bridge circuit 13B as shown in FIG. 2 as software configuration.

[0033] Steering torque signal TS output from steering torque sensor 5 is converted into digital signal to be input to target current value set unit 12A of controller 12, and vehicle speed signal VP output from vehicle speed sensor 11 is input to target current value set unit 12A. Target current value set unit 12A searches target current value signal IT to generate basic characteristic assist steering torque, increased with increase of steering torque of steering system and reduced with acceleration, in electric motor 10 from data area being steering torque signal TS and vehicle speed signal VP as address. Target current value set unit 12A outputs searched target current value signal IT to relay failure deal correction unit 12B.

[0034] Failure detection signal NG output from relay failure detection unit 12F and ON/OFF ignition signal IG output from ignition switch 15 other than target current value signal IT are input to relay failure deal correction unit 12B. Relay failure deal correction unit 12B applies loweringly correction to target current value signal IT based on failure detection signal NG from relay failure detection unit 12F and outputs correction target current value signal IT′ to deviation calculation unit 12C.

[0035] Correction target current value signal IT′ from relay failure deal correction unit 12B is input to deviation calculation unit 12C and motor current signal IM output from current sensor 14 is converted into digital signal to be input to deviation calculation unit 12C. Deviation calculation unit 12C calculates deviation of both correction target current value signal IT′ and motor current signal IM, and outputs deviation signal ΔI to PID control unit 12D.

[0036] PID control unit 12D outputs driving control signal SO to control driving of electric motor 10 to PWM signal generation unit 12E in order that deviation converges into zero by processing proportion (P), integration (I), and differential (D) etc. with deviation signal ΔI. And PWM signal generation unit 12E generates PWM signal VO to PWM drive and outputs PWM signal VO to gate driving circuit 13A of electric motor driving circuit 13 as control signal. (Here: PWM shows pulse width modulation.)

[0037] ON/OFF ignition signal IG output from ignition switch 15 is input to relay failure detection unit 12F and motor current signal IM output from current sensor 14 is converted into digital signal to be input to relay failure detection unit 12F. Relay failure detection unit 12F has function of outputting ON/OFF signal to relay circuit of power system and fail safe system, function of outputting gate signal for relay check CS to bridge circuit 13B, and function of detecting ON failure in relay circuit of power system based on motor current signal IM to output failure detection signal NG to relay failure deal correction unit 12B.

[0038] On the other hand, PWM signal VO from PWM signal generation unit 12E is input to gate driving circuit 13A in electric motor driving circuit 13 as control signal and gate signal for relay check CS from relay failure detection unit 12F is input thereto. Gate driving circuit 13A switch-drives bridge circuit 13B based on PWM signal VO at any other than case where gate signal for relay check CS is input as mentioned hereinafter.

[0039] Bridge circuit 13B of electric motor driving circuit 13 includes four electric field effect transistors for configuring bridge circuit between power supply with direct current 12V (battery mounted on vehicle) and electric motor 10, namely, four power FETs (Field Effect Transistors) T1, T2, T3, T4 as shown in FIG. 3. And each power FET (T1, T2) mutually connected against power supply in series is connected to power supply via relay circuit of power system 13C. And each power FET (T1,T3) mutually connected against electric motor 10 in series is connected to electric motor 10 via relay circuit of fail safe system 13D.

[0040] Here, once gate driving circuit 13A inputs gate signal for relay check CS from relay failure detecting unit 12F, it priority-outputs ON signal to gates G1, G3 of power FETs (T1, T3) and OFF signal to gates G2, G4 of other power FETs (T2, T4). And gate driving circuit 13A switch-drives bridge circuit 13B based on PWM signal VO from PWM signal generation unit 12E at any other than in case where gate signal for relay check CS is input. That is, gate driving circuit 13A outputs driving signal according to level of deviation signal ΔI to either gates G1 or G2 of power FETs (T1, T2) in accordance with polarity of deviation signal Δ I and outputs OFF signal to the other (the other gate to which signal is not output). At that time, it outputs ON signal to either gates G3 or G4 of power EFTs (T3, T4) and OFF signal to the other. For example, when driving signal is output to gate G1 of power FET (T1), ON signal is output to gate G4 of power FET (T4) and OFF signal is output to gates G2, G3 of the other power FETs (T2, T3).

[0041] In electric power steering apparatus, relay failure deal correction unit 12B comprises number of IG ON times count unit 12B1, correction coefficient set unit 12B2, and correction coefficient multiplication unit 12B3 as shown in FIG. 4. And relay failure detection unit 12F comprises switching signal output section 12F1, relay driving section 12F2, gate signal for relay check output section 12F3 and failure detection section 12F4. Function for each unit of relay failure detection unit 12F and relay failure deal correction unit 12B will sequentially be described below.

[0042] Ignition signal IG from ignition switch 15 is input to switching signal output section 12F1 of relay failure detection unit 12F. Once ON signal as ignition signal IG is input to switching signal output section 12F1, switching signal output section 12F1 outputs H signal for only predetermined time of initial check and then outputs L signal.

[0043] H or L signal from switching signal output section 12F1 is input to relay driving section 12F2. Relay driving section 12F2 outputs OFF signal for relay check to relay circuit of power system 13C (See FIG. 3) during initial check that H signal is input. And once L signal is input to relay driving section 12F2, it stops outputting OFF signal for relay check. And relay driving section 12F2 outputs ON signal when each power FET (T1, T2, T3, T4) is in normal mode and OFF signal when each power FET (T1, T2, T3, T4) fails to relay circuit of power system 13C and relay circuit of fail safe system 13D based on signal from unshown FET failure detection unit for detecting failure of power FET (T1, T2, T3, T4) in bridge circuit 13B.

[0044] Signal H or L from switching signal output section 12F1 is input to gate signal for relay check output section 12F3. Gate signal for relay check output section 12F3 outputs gate signal for relay check CS to gate driving circuit 13A (See FIG. 2) during initial check that H signal is input. And once L signal is input, gate signal for relay check output section 12F3 stops outputting gate signal for relay check CS.

[0045] H or L signal from switching signal output section 12F1 and motor current signal IM from current sensor 14 are input to failure detection section 12F4. Failure detection section 12F4 judges whether motor current signal IM is zero or not during initial check that H signal is input, and assumes that relay circuit of power system 13C is ON failure when motor current signal IM is not zero so as to output failure detection signal NG to number of IG ON times count section 12B1 in relay failure deal correction unit 12B.

[0046] On the other hand, failure detection signal NG and ignition signal IG are input to number of IG ON times count section 12B1 in relay failure deal correction unit 12B. After failure detection signal NG is input to number of IG ON times count section 12B1, number of IG ON times count section 12B1 accumulatively counts number of input times of ON signal of ignition signal IG. Signal of number of accumulation input times N of ON signal of ignition signal IG output from number of IG ON times count section 12B1 is input to correction coefficient set section 12B2. Correction coefficient set section 12B2 stores correction coefficient K at data area where number of accumulation input times N area is addressed and instantaneously searches correction coefficient K corresponding thereto by inputting signal of number of accumulation input times N. Searched signal of correction coefficient K is output to correction coefficient multiplication section 12B3. Correction coefficient K is decimal value of less than one and relating correction coefficient K to number of accumulation input times N is set to, for example, (N=0, K=1.00) (N=1, K=0.70) (N=2, K=0.65) (N=3, K=0.60) (N=4, K=0.55) (N=5, K=0.50) (N=6, K=0.50) (N=7, K=0.50) . . . .

[0047] Target current value signal IT from target current value set unit 12A and correction coefficient signal K from correction coefficient set section 12B2 are input to correction coefficient multiplication section 12B3. Correction coefficient multiplication section 12B3 outputs value which target current value signal IT is multiplied by correction coefficient K to loweringly correct to deviation calculation unit 12C as correction target current value signal IT′.

[0048] In electric power steering apparatus configured as aforementioned in embodiment, once ignition switch 15 of vehicle turns ON by ON operation of ignition key, relay failure detection unit 12F in controller 12 shown in FIGS. 2 and 4 checks ON failure in relay circuit of power system 13C as initial check. That is, once ON signal of ignition signal IG is input to switching signal output section 12F1 (S1) as shown in flowchart of FIG. 5, switching signal output section 12F1 outputs H signal for only predetermined time of initial check (S2). And by H signal, relay driving section 12F2 outputs OFF signal for relay check to relay circuit of power system 13C (See FIG. 3) (S3) and gate signal for relay check output section 12F3 outputs gate signal for relay check CS to gate driving circuit 13A (S4).

[0049] Gate driving circuit 13A to which gate signal for relay check CS is input connects relay circuit of power system 13C to power supply to form checking circuit by outputting ON signal to gates G1, G3 of power FETs (T1, T3) in bridge circuit 13B as shown in FIG. 3 and OFF signal to gates G2, G4 of other power FETs (T2, T4). Here, although current is not sent to current sensor 14 in case where relay circuit of power system 13C is normal mode, current is sent into current sensor 14 in case of ON failure. So, failure detection section 12F4 shown in FIGS. 2 and 4 judges existence of ON failure in relay circuit of power system 13C based on motor current signal IM in current sensor 14 (S5). In judgment, failure detection section 12F4 judges that relay circuit of power system 13C is in normal mode in case where motor current signal IM is zero and procedure proceeds to step (S8). However, failure detection section 12F4 judges that ON failure arises in relay circuit of power system 13C in case where motor current signal IM is not zero and it outputs failure detection signal NG to number of IG ON times count section 12B1 in relay failure deal correction unit 12B (S6).

[0050] When relay failure detection unit 12F does not output failure detection signal NG to number of IG ON times count section 12B1, number of IG ON times count section 12B1 outputs signal of number of accumulation input times N of N=0 to correction coefficient set section 12B2 and correction coefficient K of K=1.00 to correction coefficient multiplication section 12B3. On the other hand, when relay failure detection unit 12F outputs failure detection signal NG to number of IG ON times count section 12B1, number of IG ON times count section 12B1 counts ON signal of ignition signal IG as one (S7) and outputs signal of number of accumulation input times N of N=1 to correction coefficient set section 12B2. And correction coefficient set section 12B2 sets correction coefficient K to 0.70 (S8) and outputs correction coefficient K to correction coefficient multiplication section 12B3.

[0051] Here, after predetermined time of initial check with inputting ON signal of ignition signal IG has passed, switching signal output section 12F1 outputs L signal (S10) and relay driving section 12F2 stops outputting OFF signal for relay check (S11). And relay driving section 12F2 outputs ON signal to relay circuit of power system 13C and relay circuit of fail safe system 13D based on signal from unshown FET failure detection unit for detecting failure of each power FET (T1, T2, T3, T4) in bridge circuit 13B when each power FET (T1, T2, T3, T4) is in normal mode. It outputs OFF signal to relay circuit of power system 13C and relay circuit of fail safe system 13D when failure arises. At the same time, gate signal for relay check output section 12F3 stops outputting gate signal for relay check CS (S12) and recovers gate driving circuit 13A to normal control state.

[0052] On the other hand, steering torque sensor 5 detects direction and magnitude of steering torque generated with operation of steering wheel 1 and outputs detected steering torque signal TS to controller 12 in steering system shown in FIG. 1. And vehicle speed sensor 11 detects speed of vehicle and outputs detected vehicle speed signal VP to controller 12.

[0053] In controller 12, target current value set unit 12A shown in FIG. 2 instantaneously searches target current value signal IT to generate basic characteristic assist steering torque, which is increased with increase of the steering torque of steering system and decreased with increase of vehicle speed by inputting steering torque signal TS and vehicle speed signal VP, and outputs target current value signal IT to relay failure deal correction unit 12B.

[0054] Since correction coefficient set section 12B2 shown in FIG. 4 outputs correction coefficient K of K=1.00 to correction coefficient multiplication section 12B3 when failure detection signal NG is not input from relay failure detection unit 12F, correction coefficient multiplication section 12B3 outputs target current value signal IT to deviation calculation unit 12C as correction target current value signal IT′ as it is in relay failure deal correction unit 12B. And since correction coefficient set section 12B2 outputs correction coefficient K of K=0.70 to correction coefficient multiplication section 12B3 when failure detection signal NG is input from relay failure detection unit 12F, correction coefficient multiplication section 12B3 multiplies target current value signal IT by K=0.70 to output loweringly corrected value to deviation calculation unit 12C as correction target current value signal IT′.

[0055] Then, deviation calculation unit 12C outputs deviation signal ΔI of both correction target current value signal IT′ from relay failure deal correction unit 12B and motor current signal IM from current sensor 14 to PID control unit 12D, PID control unit 12D outputs driving control signal SO to converge deviation into zero to PWM signal generation unit 12E, and PWM signal generation unit 12E outputs PWM signal VO based on driving control signal SO to electric motor driving circuit 13. And in steering system shown in FIG. 1, assist steering torque based on correction target current value signal IT′ is applied to rack shaft 6C of rack/pinion mechanism 6 via ball screw mechanism 9 by rotating driving electric motor 10 based on PWM signal VO, which is control signal from controller 12, and thereby steering force of steering wheel 1 is reduced.

[0056] Here, as mentioned above, relay failure deal correction unit 12B outputs target current value signal IT to deviation calculation unit 12C as correction IT′ as it is in case where relay circuit of power system 13C is in normal mode in electric power steering apparatus. In contrast, relay failure deal correction unit 12B outputs correction target current value signal IT′ loweringly corrected by multiplying target current value signal IT by K=0. 70 to deviation calculation unit 12C in case where ON failure arises in relay circuit of power system 13C.

[0057] That is, driving of steering wheel 10 is controlled based on lower correction target current value signal IT′ at the time of failure than correction target current value signal at the time of normal mode in case where ON failure arises when compared with case where relay circuit of power system 13C is in normal mode in electric power steering apparatus. As a result of this, assist steering torque applied to steering system by steering wheel 10 is loweringly corrected and steering becomes possible by steering wheel 1 even for vehicle of large load applied to front Shaft. And in this case, since steering torque becomes heavier than torque at the time of normal mode, it is possible to inform driver to repair vehicle.

[0058] After ON failure in relay circuit of power system 13C is detected, number of IG ON times count section 12B1 of relay failure deal correction unit 12B accumulately counts number of input times of ON signal of ignition signal IG for every restart of vehicle in electric power steering apparatus. And correction coefficient K which correction coefficient set section 12B2 sets according to number of accumulation input times N which number of IG ON times count section 12B1 counts is gradually decreased. That is, after ON failure in relay circuit of power system 13C is detected, steering torque of steering wheel 1 becomes gradually heavier for every restart of vehicle. Therefore, it is possible to well inform driver to repair vehicle.

[0059] Further, relay failure deal correction unit 12B may be configured as shown in FIG. 6 in electric power steering apparatus of present invention. That is, it may be configured by number of IG ON times count section 12B1 similar to aforementioned number of IG ON times count section 12B1, and limiting current value set section 12B4 and maximum current value limiting section 12B5 mentioned hereinafter.

[0060] Limiting current value set section 12B4 stores limiting current value LIM in data area where number of accumulation input times N of ON signal of ignition signal IG which number of IG ON times count section 12B1 outputs as signal is addressed and instantaneously searches limiting current value LIM corresponding to input signal by inputting signal of number of accumulation input times N. And limiting current value set section 12B4 outputs searched signal of limiting current value LIM to maximum current value limiting section 12B5. Maximum current value acceptable in each power FET (T1, T2, T3, T4) in bridge circuit 13B is set and limiting current value LIM is gradually decreased. Relating number of accumulation input times N to limiting current value LIM is set to, for example, (N=0, LIM=80 A) (N=1, LIM=60 A) (N=2, LIM=55%) (N=3, LIM=50%) (N=4, LIM=45%) (N=5, LIM=40%) (N=6, LIM=40%) (N=7, LIM=40%).

[0061] Maximum current value limiting section 12B5 inputs target current value signal IT from target current value set unit 12A and limiting current value LIM from limiting current value set section, and outputs lower value to deviation calculation unit 12C as correction target current value signal IT′ by comparing input target current value signal IT with input limiting current value LIM.

[0062] Relay failure deal correction unit 12B configured like this loweringly corrects maximum value of assist steering torque to be applied to steering system within limit, which is not more than limiting current value LIM of maximum value of target current value signal IT which target current value set unit 12A sets in case where ON failure arises in relay circuit of power system 13C. And relay failure deal correction unit 12B gradually lowers limiting current value LIM for every restart of vehicle to loweringly correct maximum value of assist steering torque gradually after ON failure arises in relay circuit of power system 13C. Functional effect similar to electric power steering apparatus of aforementioned one embodiment can be obtained.

[0063] As described above, once ignition switch turns ON, relay circuit turns ON according to instruction of controller and electric motor driving circuit and electric motor are connected to power supply. And steering torque sensor detects steering torque of steering system with operation of steering wheel and controller outputs control signal based on steering torque to electric motor driving circuit to control assist steering torque to be applied to steering system with electric motor. At that time, when relay failure detection unit of controller detects ON failure in relay circuit, relay failure deal correction unit loweringly corrects maximum value of assist steering torque with electric motor based on failure detection signal.

[0064] Therefore, according to electric power steering apparatus of present invention, manual steering by steering wheel becomes possible even for vehicle of large load applied to front shaft when ON failure arises in relay circuit. And since steering torque of steering wheel becomes heavier than torque at the time of normal mode, it is possible to inform driver to repair vehicle.

[0065] In electric power steering apparatus of present invention, since steering torque of steering wheel becomes gradually heavier for every restart of vehicle after ON failure arises in relay circuit when relay failure deal correction unit is configured such that amount of loweringly corrected assist steering torque is gradually increased according to number of accumulation input times of ON signal of ignition switch after ON failure arises in relay circuit, it is possible to well inform driver to repair vehicle. 

What is claimed is:
 1. An electric power steering apparatus comprising: (1) an electric motor for applying an assist steering torque to the steering system of a vehicle; (2) an electric motor driving circuit for driving the electric motor; (3) a steering torque sensor for detecting the steering torque of the steering system; (4) a controller for outputting a control signal based on a detection signal of the steering torque sensor to the electric motor driving circuit to control an assist steering torque with the electric motor; and (5) a relay circuit, which is capable of switching ON/OFF according to instructions of the controller, which is provided between the electric motor driving circuit and a power supply, wherein the controller includes (a) a relay failure detection unit for detecting ON failure in the relay circuit; and (b) a relay failure deal correction unit for loweringly correcting the assist steering torque, at least the maximum value of the assist steering torque, of the electric motor based on the failure detection signal of the relay failure detection unit at the time of ON failure of the relay circuit.
 2. An electric power steering apparatus as set forth in claim 1 , wherein the relay failure deal correction unit gradually increases the amount of the loweringly corrected assist steering torque according to the number of accumulation input times of the ON signal of an ignition switch after ON failure arises in the relay circuit.
 3. An electric power steering apparatus as set forth in claim 1 , wherein the relay failure deal correction unit multiplies a signal value, the signal value being set with the controller, by a correction coefficient having a decimal value, which is less than one, to loweringly correct the maximum value of the assist steering torque based on the detection signal of the steering torque sensor.
 4. An electric power steering apparatus as set forth in claim 2 , wherein the relay failure deal correction unit multiplies a signal value, the signal value being set with the controller, by a correction coefficient having a decimal value, which is less than one, to loweringly correct the maximum value of the assist steering torque based on the detection signal of the steering torque sensor.
 5. An electric power steering apparatus as set forth in claim 1 , wherein the relay failure deal correction unit loweringly corrects the maximum value of the assist steering torque within a limit of the maximum value of a signal value which the controller sets based on the detection signal of the steering torque sensor.
 6. An electric power steering apparatus as set forth in claim 2 , wherein the relay failure deal correction unit loweringly corrects the maximum value of the assist steering torque within a limit of the maximum value of a signal value which the controller sets based on the detection signal of the steering torque sensor. 